In this article, we’ll go over some basic calculations to help you understand how solar systems are sized and how it is determined the number of panels you will need.
PLEASE NOTE: This article contains very basic equations and makes few assumptions! There are multiple variables to consider when a solar system is designed, including but not limited to:
- 12 months or more of electrical usage history (recommended but not required)
- The type of panels you buy and how many watts they are rated for
- The average number of sun hours (consistent light hitting the panels)
- Panel location on your roof (a south-facing roof plane is ideal)
- Any shading from trees or other obstructions
- Geographical location and climate
- Season & time of year
Our LGCY energy consultants have the tools, technology and experience when it comes to designing solar systems to match your usage history and fit your home correctly. They can present you with multiple options and help you plan for current and future energy needs beyond basic calculations.
Contact us today for a consultation and see what solar can do for you!
Example Scenario
For the sake of understanding how these calculations work, we’re going to make some very basic and broad assumptions:
- We will use 11,000 kW (kilowatts) as the average energy usage from the previous year, which breaks down to 917 kW per month or 31 kW per day on average (the average national consumption for 2018 was 10,972 according to the US EIA)
- We will assume you get an average of at least five hours of consistent sunlight (sun hours) where you are located; your actual sun hours may vary – click here for maps from the NREL to find out more
- We will use both basic (320 watt) and premium (365 watt) panels to compare different system sizes, and
- We will factor both 80% offset and 100% offset energy consumption with the above equipment as a means to compare and contrast different options
“System size” is determined by multiplying the number of panels by the watt rating of the panel (ex. 350w x 15 panels = 5.25 kW system). The average system size that LGCY power installs is between 7-8 kW.
Example 1: Basic Panel (320 W) with 80% Offset
80% of 31 kW or 31,000 watts is 24.8 kW or 24,800 watts (this is our target)
(320 W rated panel x 15 panels) x 5 hours of sunlight = 24,000 W (below target)
(320 W rated panel x 16 panels) x 5 hours of sunlight = 25,600 W (meets target)
(320 W rated panel x 17 panels) x 5 hours of sunlight = 27,200 W (exceeds target)
To meet or exceed 80% of your daily energy needs, you would need a system sized at 5.12 kW, consisting of 16 panels rated at 320 watts each.
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Example 2: Basic Panel (320 W) with 100% Offset
31 kW or 31,000 watts (this is our target)
(320 W rated panel x 19 panels) x 5 hours of sunlight = 30,400 W (below target)
(320 W rated panel x 20 panels) x 5 hours of sunlight = 32,000 W (meets target)
(320 W rated panel x 21 panels) x 5 hours of sunlight = 33,600 W (exceeds target)
To meet or exceed 100% of your daily energy needs, you would need a system sized at 6.40 kW, consisting of 20 panels rated at 320 watts each.
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Example 3: Premium Panel (365 W) with 80% Offset
80% of 31 kW or 31,000 watts is 24.8 kW or 24,800 watts (this is our target)
(365 W rated panel x 13 panels) x 5 hours of sunlight = 23,725 W (below target)
(365 W rated panel x 14 panels) x 5 hours of sunlight = 25,550 W (meets target)
(365 W rated panel x 15 panels) x 5 hours of sunlight = 27,375 W (exceeds target)
To meet or exceed 80% of your daily energy needs, you would need a system sized at 5.11 kW, consisting of 14 panels rated at 365 watts each.
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Example 4: Premium Panel (365 W) with 100% Offset
31 kW or 31,000 watts (this is our target)
(365 W rated panel x 16 panels) x 5 hours of sunlight = 29,200 W (below target)
(365 W rated panel x 17 panels) x 5 hours of sunlight = 31,025 W (meets target…barely!)
(365 W rated panel x 18 panels) x 5 hours of sunlight = 32,850 W (exceeds target)
To meet or exceed 100% of your daily energy needs, you would need a system sized at 6.20 kW, consisting of 17 panels rated at 365 watts each.
